Leak detection routines may be intermittently performed on a fuel system and emissions control system to confirm that the systems are not degraded. Leak detection routines may be performed while the engine is off (engine-off leak test) using engine-off natural vacuum (EONV) generated due to a change in temperature and pressure at the fuel tank following engine shutdown and/or with vacuum supplemented from a vacuum pump. If the systems are sealed from atmosphere, a pressure or vacuum will develop there within responsive to changes in ambient temperature if the systems are intact. Alternatively, leak detection routines may be performed while the engine is running by operating a vacuum pump and/or using engine intake manifold vacuum.
Such leak detection routines rely on a functional fuel tank pressure transducer (FTPT) to measure the pressure or vacuum within the fuel system. As such, the rationality of the FTPT must be periodically tested and confirmed. The FTPT may be tested for offset, to determine if a baseline output of the FTPT is accurate. One example approach for an FTPT offset test is shown by Jentz et al. in U.S. Patent Application 2015/0075251. Therein, the fuel tank is vented to atmosphere for a lengthy vehicle-off soak. If the FTPT is functional, a value within a threshold of atmospheric pressure should be output following the vehicle-off soak. A deviation from atmospheric pressure may result in a diagnostic trouble code (DTC) being set at the controller, and/or may result in the FTPT output being adjusted to compensate for any offset.
However, the inventors herein have recognized potential issues with such systems. As one example, an offset FTPT output following a vehicle-off soak may be due to factors other than FTPT degradation. For example, if the canister vent pathway is restricted or blocked, a pressure or vacuum may naturally develop in the fuel tank as ambient temperature changes. A mechanic may replace a functional FTPT due to a DTC, and may or may not discover the canister vent restriction. This may result in premature automatic shut-off events during refueling, and may prevent proper canister purging, thereby increasing vehicle emissions.
In one example, the issues described above may be addressed by a method for a fuel system wherein a fuel vapor canister is vented to an engine intake during a first condition, and wherein a restriction in a canister vent pathway is indicated, responsive to a change in a fuel tank pressure transducer output greater than a threshold. If the fuel tank pressure transducer output changes less than the threshold, degradation of the fuel tank pressure transducer is indicated. In this way, a fuel tank pressure transducer offset may be distinguished from a canister vent pathway restriction if a fuel tank pressure transducer indicates a significant pressure or vacuum in a vented fuel tank.
As one example, the fuel tank may be vented to atmosphere for the duration of a vehicle-off soak. If the fuel tank pressure transducer output is more than a threshold from atmospheric pressure, the canister purge valve is opened. If the FTPT output returns to atmospheric pressure, a canister vent pathway blockage may be inferred. If the FTPT output does not return to atmospheric pressure, FTPT degradation may be inferred. In this way, an on-board test can discern potential reasons for FTPT offset without requiring mechanical intervention.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.